This invention relates to semiconductor memory devices, and more particularly to an N-channel silicon gate MOS read only memory and a process for making it.
Storage of fixed programs in digital systems is often by MOS read only memory devices or "ROMs". ROMs are made by semiconductor manufacturers on special order, with the programming code being specified by the customer. The manufacturing process is lengthy, requiring dozens of steps, each taking up time and introducing materials handling and inventory factors. The turnaround time between receipt of the ROM code for a custom order and delivery of finished parts must be kept as short as possible. For this reason, programming should be done late in the manufacturing process, such as by the "implant through poly" method as disclosed in pending applications Ser. Nos. 890,555; 890,557; 900,549 (U.S. Pat. No. 4,230,504), and 1,571 (U.S. Pat. No. 4,294,001), filed by C-K Kuo, assigned to Texas Instruments, or U.S. Pat. No. 4,129,936. Programming the ROM by ion implant through polysilicon gates allows the program step to be done after most of the steps in the manufacturing process have been completed.
The economics of manufacture of ROMs and of mounting them on circuit boards in a system require that the number of memory bits per semiconductor chip be as high as possible. Within a few years, standard ROM sizes will exceed 1 megabit per chip, dictating that the cell size for the storage cells of the ROM be quite small. Most microprocessor and computer parts are now made by the N-channel silicon gate process because of the higher speed or shorter access time provided. The N-channel process has not been favorable to layout of ROM cells of small size in a structure compatible with implant programming. The above-mentioned applications or patent show devices which require non-standard N-channel processing, or result in excessive cell size.
It is the principal object of this invention to provide a semiconductor permanent store memory cell of small size made by standard processing which may be programmed at a late stage in the manufacturing process. Another object is to provide a small-area MOS ROM cell which is made by a process compatible with standard N-channel silicon gate manufacturing techniques and is programmable after the metal interconnections have been applied and patterned.